Safest way to de-energize a tube head before digging in.
Okay, whats the safest and fastest way to de-energize a tube head and should I de-energize all the capacitors and the transformer? If I do power down the circuits how to I keep the capacitors from surging with transiant overvoltage when powering up?
He asked a simple question that doesn't need to be answered with a question.
to the OP:
You didn't mention how old your amp is. That's important because if it's a tube amp of recent manufacture, safety standards require there to be bleed-down resistors on the power supply caps that are appropritely sized to drain down the amp within 30 seconds of power-off. They are intended to automatically makes the amp safe. If you have a really old amp that doesn't have bleed-down resistors then you'll have to do things manually.
How experienced are you?
The safest thing to do if you're not an experienced tech: unplug the amp while it's turned on and you're playing it, and discharge the caps with a musical load. Let it sit overnight, without going inside of the chassis. Or turn the amp's power switch off but leave the standby engaged while you're playing to draw down the power. Keeping a musical load on the circuit will help to run down the energy in the caps.
Now check the power supply voltages while staying outside of the amp. The safest way to check for PSU discharge is to pull one of the power tubes and use a DC volt meter to test the voltage between the pin that corresponds to the plate (anode) of the power tube and chassis ground. When the PSU is charged that voltage will be several hundred volts DC. You want to see in measure as close to 0 VDC as possible.
Now if you're comfortable working inside of the chassis:
The textbook answer is with a discharge probe. I built mine using a 5W to 10W 10k resistor on a pair of insulated leads with insulated alligator clips on the ends. Clip across the B+ rail and ground to let the B+ discharge into the resistor. Check the voltage across the resistor with your volt meter and wait until the voltage on the supply rail is close to 0VDC.
Amps that have "bleeder" resistors across the PSU caps (or balancing resistors across a totem pole) will do this automatically, but the time constant for discharge depends on the value of the cap and the value of the resistor. If the bleeders are 110k to 220k that could take a long time. Fender uses 33k in some of their bigger amps for this reason. A 10k resistor with a suitable wattage rating that's built into a dedicated discharge probe would be a lot faster.
Thanks man, I already assumed most of what you just confirmed so I feel pretty safe. It's always good to get a second opinion.
To complete your question the "transformers" do not store energy, they are "de-energized" by turning off the power switch. Which makes me concerned that you should not be working around high voltage circuits without doing some study time first? ;)
Does a transformer have capacitance after the primary is switched off? I only know enough to know I don't know enough to want to mess with 500V.
Be afraid OP. One slipup can kill you stone dead. If you don't absolutely know what's what keep studying.
Bstring types faster than me. He also knows what's what.
Safest way is to fabricate your own bleed resistor and manually bleed the caps yourself, as spelled out in the remaining part of the post.
An old-timer's safety rule when working on an energized unit is to work with one hand and always keep the other hand in your back pocket.
Better yet is to work on a de-energized unit. To be fair, the amp is no longer a high voltage circuit when it's unplugged from the wall and once the PSU has been discharged. ;) Once you've successfully discharged the power supply then you're totally safe to take passive measurements. The danger only comes along when the unit gets plugged back in.
In the big scheme of things, the vast majority of repairs that you'll need to do on a tube amp can be diagnosed by taking voltage measurements at the tube sockets, and by making a voltage table for analysis. It's not all that often that you need to go prodding around inside of the amp with test leads. Usually you can get all of the measurements you need at the sockets, without removing the chassis from it's container, so that you're never exposed to what's inside of the amp.
If you're not experienced -- play it safe -- pull the tubes and draw up a voltage table using your volt meter to meter the potentials at the tube sockets, rather than going inside of the amp. Doing it that way you don't have to worry about discharging the amp from the inside. Even if you are experienced, there's a lot to be said for just recording the pin voltages onto a table, and then shutting down/bleeding down the amp while you take some time to compare the voltage readings to the schematic.
It isn't just the power supply electrolytic caps (those that have a + or - on them) but also the bias supply cap or caps. Look for any capacitor with a polarity on it and discharge it.
Just keep in mind that a charged capacitor has enough energy in it to take a bite out of a steel screwdriver. So be careful.
If your amp sits for a while unused, the electrolytic caps will discharge. So discharging your caps to work on your amp will not be an issue. Voltage levels in capacitors will surge when powering up. It is possible, depending on the design that you can exceed the rated voltage of the cap. Good caps are designed to take an over voltage when charging without any ill effects. Marginally rated, budget caps may not stand up over time as well as better quality caps. It means is that you will have to replace the caps sooner. All electrolytic caps need to be replaced with time. It is inherent in their design.
Assuming that OP knows what he's doing and also assuming an old amp without the bleeder resistors.
Leave the bleeder resistor attached to the rails/capacitor leads for as long as You work inside the amp.
Electrolytic caps have a tendency to "remember" the state they're in and some of the charge returns after a while.
Do use bright coloured shrink tubing when building the bleeder probe to reduce the possibility to accidentally leaving it there when powering up ;).
Just take a rubber handled screwdriver and bridge the capacitors to ground. That's discharge 'em pretty quick.
^^ Screwdriver = Very bad advice.
More like a bad joke I think.
Use your tongue and just taste everything in the chassis. Keep both hands on your face too, and preferably attach as many alligator clips to your nose and ears so they dangle freely around inside. Boom!
Seriously though - if you let the tubes warm up properly (as if you were about to play) most of the current is drained through the tubes. Always double check the voltages and do NOT rely on just "well he said that I did that it must be fine" - take serious consideration. Do research all over the web, look up "tube amp capacitor discharge" or something along those lines and be blown away by the wealth of info regarding proper safety.
There's lots of good info here now - a resistor for discharge, keeping it connected while working or changing components, keeping one hand behind your back, etc.
Wear rubber soled shoes, be on carpet, be aware of where your hand is, never rush.
Measure the positive side of the electrolytics, keep the black probe of the DMM to ground, connect an alligator clip to the red probe and the other end of that clip to a long wooden skewer and use that wooden skewer as a "poker" to connect to the electrolytic for safe(r) voltage readings.
It's late so I be ramblin' but I love this subject and I'm learning as you are too! LET'S GGOOOOOO
I would say don't do it. It's not worth the risk.
If you are still going through with it contact Dr. Tube.
He knows the spots.
And when you are going to do it try to have a friend with you with a broom in hand.
So he can save your ass when you get electrocuted.
I'm amazed at how quickly a guy is made feel unwelcome because he has less knowledge of how or when a transformer is safe to work around. System plugged in, power on, transformer is hot, system unplugged switch on or off, system is de-energized, but is it safe plugged in with power switch off? Doesn't the system need to be hot to get ac and dc voltages? Thanks for the help from those of you who have the decency to answer my questions without being arrogant.
I hope we are not sounding like we are making you feel unwelcome, but unfortunately we don't have any way of knowing someone's level of electrical expertise when they post asking for some technical advice. We've got people that post on here that frankly have no business opening up an amplifier, and on the other extreme we have people on here that design and build amplifiers commercially.
Also, we don't now what vintage amp you're working on. People post on here with ancient "hot chassis" amps more than 50 years old, other folks are working on amps with the latest and greatest technology (even with a tube amp). So we've got to assume the worst, that it's an older amp which can still hold charge on the caps for a LONG time.
As an example, I built a benchtop power supply using a voltage doubler and a B+ of around 300 volts DC. Lots of stored energy in those caps and no bleeder resistor, it was purely for benchtop experimentation. Many months after unplugging the power supply, there was still enough charge in those caps to yield a healthy spark when I shorted + to ground.
I routinely work on amps with them still plugged in and powered up, how else can one measure voltages on certain spots? However, a certain level of caution and awareness must be exercised. You don't have to be deathly afraid of working on the amp hot provided you are aware of what is energized, what isn't, where you can touch with your probes, where you should be cautious.
Unfortunately, though, as I said at the outset of this monologue, we do have people opening up their amps without a clue and doing the strangest things, so that's why we must at times sound a little overbearing.
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